A living hinge is a thin, flexible hinge connecting two relatively rigid parts. It is usually made from the same material as the rigid parts. It may be used to join rigid parts of a container, allowing them to bend along the line of the hinge. Polypropylene (PP) has traditionally dominated the living-hinge dispensing closure market as it is easily processed and has good hinge durability characteristics. Additionally, it is widely available and historically had favorable economics relative to polyethylene when utilized to these ends.
These attributes coupled with PP's overall balance of properties make it a frequent choice for living-hinge closure applications, as well as many molding applications.
In addition, it has become more desirable to have a closure that is made from the same type of polymer as the bottle to enable recycling of the entire container. Further, polyethylene has not always been a suitable replacement of PP due to poor living hinge durability, especially when a high melt index is needed for polyethylene to have good processability. That is, polyethylene has not necessarily proven to be mechanically strong enough to last a large number of flexing cycles.
Accordingly, it may be desirable to produce polyethylene compositions having improved processability and/or longer living hinge durability.